Method of providing conductive tracks on a printed circuit and apparatus for use in carrying out the method

ABSTRACT

A method of electroplating comprises providing an anode current for a target, applying an electron beam to the surface of a target and passing electrolyte between said target and anode, thereby to deposit material on said target. An electron beam gun directs an electron beam onto web while anode provides a current thereby depositing material on the web.

RELATED APPLICATION

[0001] This is a continuation-in-part of U.S. Ser. No. 09/679,577 filedOct. 5, 2000, which is a continuation of International Application No.PCT/GB99/00890, with an international filing date of Apr. 6, 1999, whichis based on British Patent Application No. 9807280.4, filed Apr. 6,1998.

FIELD OF THE INVENTION

[0002] This invention is concerned with a method of providing conductivetracks on a printed circuit and an apparatus suitable for use incarrying out the method.

BACKGROUND

[0003] A well known and convenient way of producing printed circuitsincludes the printing of electrically conductive tracks on a substrate,for example, using screen printing techniques. The tracks are printedusing an electrically conductive ink which typically comprises apolymeric material having electrically conductive particles, forexample, copper, silver or other suitable metal, dispersed in thepolymeric composition. The polymeric materials are typically cured to asolid condition by subjecting them to radiation, for example, infra-redradiation or ultra-violet light.

[0004] Although the conductive inks which are used have a sufficientelectrical conductivity for use in certain circumstances, the electricalconductivity has in no case been as great as the electrical conductivityof copper or other highly conductive metals. Even the conductive inkswith the best performance have electrical conductivities which in mostcases are only one tenth of the electrical conductivity of copper. Ithas been proposed to enhance the conductivity of the conductive tracksas continuous printed circuits by electro-plating the tracks with asuitable layer of metal, for example, copper but, in order to carry outelectro-plating, it is necessary to have a continuous electricalcircuit: that is not conveniently possible where the printed conductivetracks of a printed circuit are discrete and are not connected with oneanother. Furthermore, electro-plating has required the immersion of thesubstrate carrying the printed conductive tracks in a bath of suitableelectro-plating solution: that technique restricts the substrates whichcan be subjected to such an immersion electro-plating technique—forexample—paper based substrates are generally unsuitable as they willtend to be attacked and softened by the electro-plating solution. It hasalso been proposed to provide a more conductive coating on conductivetracks of printed inks by electroless plating. Electroless platinginvolves the use of plating solutions which are less stable than thosecommonly used in electro-plating and the process is less readilycontrolled. Furthermore, electroless plating still requires thesubstrate to be immersed in the plating solution with the consequentpossibilities of attack of the substrate as well as being restricted inthe thickness of deposit.

[0005] Thus, it would be advantageous to provide an improved method ofproviding conductive tracks on a printed circuit by electro-plating andto provide an improved printed circuit.

SUMMARY OF THE INVENTION

[0006] In one aspect, the invention comprises an electro-plating cellwherein the cathode connection is formed by an electron beam. Theinvention also provides a method of electro-plating comprising providingan anode current, applying an electron beam to the surface of a targetand passing electrolyte between the target and anode, to thereby depositmaterial on the target.

[0007] The method may include any one or more of the following features:

[0008] Ionizing the target by treating with a scanning electron beam tocreate an opposite polarity to the polarity of the anode;

[0009] Providing an electrically conductive layer at the surface of thetarget;

[0010] Providing a layer of material of greater density than theneighbouring material;

[0011] Providing material suitable for forming a lattice with thematerial being deposited;

[0012] Focusing the electron beam by tuning of voltages applied withinthe electron gun;

[0013] Spreading of the electron beam by tuning of voltages appliedwithin the electron gun;

[0014] Varying the position of the beam on the target;

[0015] Directing a plurality of electron beams onto one or more regionsof the target;

[0016] Directing two electron beams onto a target from opposite sides ofthe target;

[0017] Directing and focusing two electron beams, from opposite sides ofa target, to collide in the vicinity of a target;

[0018] Directing two electron beams from one side of the target, anddirecting one electron beam from the other side of the target;

[0019] Directing an electron beam from the region of the source of theanode current.

[0020] Also, the invention provides electro-plating apparatus comprisingan anode to provide current at a target, means to apply an electron beamto the target and means to pass electrolyte between the target andanode, to thereby deposit material on the target.

[0021] The electro-plating apparatus may include any one or more of thefollowing features:

[0022] An electro-plating cell wherein the cathode connection is formedby an electron beam;

[0023] A scanning electron beam gun to ionize a target;

[0024] Means to focus the electron beam by tuning of voltages appliedwithin an electron gun;

[0025] Means to spread the electron beam by tuning of voltages appliedwithin an electron gun;

[0026] Means to vary the position of the electron beam at the target;

[0027] An electron beam source to direct a plurality of electron beamsto one or more regions of the target;

[0028] An electron beam source to direct at least two electron beamswhich are on opposite sides of the target;

[0029] Two electron beam guns positioned on opposite sides of the targetfor the direction and focusing of beams for collision at the target;

[0030] One electron beam source to direct an electron beam from one sideof the target, and two electron beam sources to direct two electronbeams from the other side of the target;

[0031] An electron beam source is located in the anode.

[0032] The invention also provides a computer product directly loadableinto the internal memory of a digital computer, comprising software codeportions for performing the steps of a method of the present inventionwhen the product is run on a computer.

[0033] The invention also provides such a computer program product on acarrier, and to electronic distribution or storage of such a computerprogram product on a carrier.

[0034] In one aspect, the invention relates to a method of providingconductive tracks on a printed circuit by electro-plating conductivetracks which have been produced by printing them onto a substrate,comprising coating a substrate carrying the printed tracks with anelectro-plating solution using a tool which provides a first electrodeof an electro-plating circuit and with a second electrode provided bythe tracks which are to be electroplated.

[0035] In carrying out aspects of the method in accordance with theinvention ink forming the conductive tracks preferably comprises a curedpolymer composition loaded with electrically conductive particles.

[0036] Conveniently in carrying out a method in accordance with aspectsof the invention, a first pole of an electro-plating circuit isconnected to the first electrode and a second, opposite pole of theplating circuit is connected to the second electrode.

[0037] Preferably in carrying out aspects of the method in accordancewith the invention use is made of apparatus including a tool whichcomprises an absorptive member in which the plating solution can becarried, in which the first electrode is in electrical connection withplating solution carried by the absorptive member, and in which thecoating of plating solution is applied to the substrate by wiping theabsorptive member over the substrate. Conveniently, the tool alsocomprises a second electrode, electrically insulated from the firstelectrode and spaced from the absorptive member, in which the secondelectrode is adapted to be wiped across the surface of the substrate asthe absorptive member is wiped across the surface of the substrate.

[0038] In another aspect of the method in accordance with the inventionthe conductive tracks may be treated with a scanning electron beam toionize the tracks and create an opposite polarity to the polarity of thefirst electrode.

[0039] Conveniently in carrying out aspects of the method in accordancewith the invention the plating solution comprises copper sulphate.However, any suitable electrode-plating solution which can be carried bythe absorptive member may be used. In carrying out a method inaccordance with the invention the conductive tracks are convenientlycoated with plating solution sufficient to deposit a layer of copper onthe tracks which is of a desired thickness, typically about 2 microns inthickness.

[0040] In another aspect the invention, there is provided a toolsuitable for use in electro-plating electrically conductive regions of asubstrate comprising an absorptive member in which the plating solutioncan be carried, a first electrode adapted to make electrical contactwith a plating solution carried by the absorptive member, and at leastone second electrode electrically insulated from the first electrode andspaced from the absorptive member, the second electrode being sopositioned that as the absorptive member is wiped across a surface of asubstrate the second electrode can be wiped across the surface of thesubstrate.

[0041] In carrying out aspects of the method in accordance with theinvention, use is preferably made of a tool in accordance with theinvention.

[0042] The absorptive member of a tool in accordance with the inventionmay be provided by any suitable means within which the plating solutionmay be absorbed—for example the absorptive member may comprise a brushor a flexible foam material having interconnected pores.

[0043] Suitably a tool in accordance with the invention comprises meansto feed a supply of plating solution to the absorptive member.

[0044] A preferred tool in accordance with the invention comprises twoelectrically connected second electrodes mounted such that as theabsorptive member is wiped across the surface of a substrate one of thesecond electrodes leads the absorptive member and the other secondelectrode trails the absorptive member. Conveniently, the or each secondelectrode is provided by a flexed electrically conductive blade, forexample a flexible metal blade, e.g., of copper.

[0045] In another aspect the invention may be considered to provide aprinted circuit comprising a plurality of discrete conductive tracks,each track comprising a layer of a cured electrically conductive ink onan electrically insulating substrate and a layer of conductive metaldeposited on the cured ink by electro-plating.

[0046] In carrying out aspects of the method in accordance with theinvention it is not necessary to immerse the substrate carrying theconductive tracks in a plating bath. The quantity of plating solutionwhich comes into contact with the substrate is very small and the methodcan in effect be regarded as a substantially dry electroplating method.Consequently, it is possible to electro-plate tracks on substrates whichcannot be electro-plated by an immersion system. Furthermore, incarrying out a preferred method in accordance with the invention, it isnot necessary that the conductive tracks be continuous, the use of atool in accordance with the invention or the scanning electron beamtechnique enables discreet electrically conductive tracks to beelectro-plated. A method in accordance with the invention can becontrolled to provide an accurate plating thickness: the thickness of alayer applied by electro-plating is a function of current and time forwhich the electro-plating solution is in contact with the region to beplated. It is therefore possible to electro-plate regions of a printedcircuit to provide electro-plated regions of different thickness therebypermitting adjustment of the resistance of regions of the printedcircuit to provide resistors of the circuit. Such a system isparticularly conveniently carried out using the scanning electron beamwhich can be targeted accurately on to a specific conductive track andmoved rapidly to achieve the desired conductivity.

[0047] Although in carrying out aspects of the method in accordance withthe invention the tool by which the plating solution is applied may behand held, it is preferable to mount the tool in a suitable machinewhich may wipe the tool across the surface of the substrate carrying theconductive tracks.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order that the invention may more readily be understood, adescription is now given, by way of example only, reference being madeto the accompanying drawings, in which:

[0049]FIG. 1 is a schematic side view of a prior art electro-platingstation;

[0050]FIG. 2 is a side view of an electro-plating station of the presentinvention;

[0051]FIG. 3 is an enlarged view of the web in FIG. 1;

[0052]FIG. 4 shows an alternative electro-plating station;

[0053]FIG. 5 shows a further alternative electro-plating station;

[0054]FIG. 6 shows a further alternative electro-plating station;

[0055]FIG. 7 shows a printed circuit; and

[0056]FIG. 8 shows an electro-plating head.

DETAILED DESCRIPTION OF THE INVENTION

[0057]FIG. 1 illustrates schematically a conventional electro-platingstation 1 having an anode 2 over which web 3 passes. Seals 4 and 5define the extremities of the inlet and outlet channels 6 and 7 for flowof electrolyte to and away from the electro-plating area of the web 3.Rows of fingers 8 and 9 provide the cathode connection.

[0058] Also the separation distance between rows of fingers 8 and 9defines the smallest feature size Δfs which can be achieved with station1, in that one or other of fingers 8 or 9 must be touching the featureto be plated. Typically, this distance is of the order of about 4×10⁻²meters.

[0059] There is shown in FIG. 2 an electro-plating station S of theinvention comprising a head 20 having an anode 21 of thickness 6×10⁻³meters, to one side of which there is located an electrically-neutralwall 23 with an arcute upper section 24. A passageway 25 is formedbetween anode 21 and wall 23 for electrolyte 26 being a solution ofcopper sulphate flowing at a speed of 4 litres sec⁻¹.

[0060] A web 27 of material of width 1 meter on which copper is to beselectively deposited, is moved at a substantially uniform speed of 0.2meters min⁻¹ over head 20. Anode 21, beam 35 (see FIG. 3) andelectrolyte 26 allow a current density of 1.27×10⁵ Amps meter⁻²resulting in deposition of copper to a thickness of 2×10⁻⁶ meters. Thespeed of movement of the web 27 is maintained constant, typical speedsbeing up to or greater than about 6 meters min⁻¹.

[0061] Guide 24 of wall 23 is shaped to enhance and maximize theproduction of streamwise vortices. Anode 21 is 6×10⁻³ meters wide at itstop surface 28 and is 1.1 meter long to accommodate the width of web 27.

[0062] Serrations (not shown) are provided on the anode 21 in the areaof top surface 28, and a mesh 30 located in passageway 25 alsocontribute to the generation of these vortices.

[0063] Electro-plating station S has a mesh 31 located at a throatsection 32 of passageway 25 shortly before the start of the guide 24,the mesh being a polyester mesh N8 type of 34.6 threads 10⁻² meters witha thread diameter of 1.04×10⁻⁴ meters giving a maximum open area of 38%.

[0064] Directly above top surface 28 of anode 21, and separated from topsurface 28 by a minimal clearance to accommodate web 27 and electrolyteflow, is located the quartz outlet window 33 of an electron beam gun 34which can provide electron beams with currents in the range of from lessthan about 10⁻⁶ Ampere to about one Ampere or greater. The beam may havea radius of as little as about 1×10⁻⁸ meter and as large as about 3×10⁻²meter. The diameter of the beam is normally larger for higher beamcurrents. The cross-section of the beam need not be circular, but mayhave an elongated format. For example, a typical beam may have adiameter of about 1×10⁻³ meter with a current of about 1×10⁻¹ Ampere.

[0065] In operation, electro-beam gun 34 directs a focused scanning beam35 of electrons to a static area of typically about 1×10⁻⁶ meter 2 inlayer 36 of thickness varying from less than about 1×10⁻⁶ meter togreater than about 5×10⁻⁵ meter on web 27 (see FIG. 3) as it passesclose by top surface 28 of anode 21.

[0066] Layer 36 may be electrically-conductive, e.g., a silver-loadedink, vapor-deposited copper or carbon-loaded ink to form a seed layer.Alternatively, it can be of other materials, typically more dense thanthe remainder of the web 27, and preferably suitable to form a latticewith the material being deposited (typically being copper) or any othermaterially used in electro-plating.

[0067] In another form, there may be no distinctive layer 36, but ratherthe presence of the electrolyte stream is sufficient to provide aninitial basis for deposition of material. A rough surface of the web maycontribute to the initial deposition of material.

[0068] The electron beam 35 from gun 34 can be focused or spread byappropriate tuning of the voltages applied within gun 34, and likewisethe position of the beam can be moved in any combination of the two axesrelative to the plane of web 27. Thus, by suitable varying of the sizeand position of the material deposited on moving web 27, actual“writing” onto the web can be achieved.

[0069]FIG. 4 shows another electro-plating station 50 which has threeelectro-beam guns 51, 52 53, one gun 51 being located above web 54 andon the opposite side of it to anode 55, and two guns 52 and 53 below theweb 54 and on opposing sides of anode 55. In a variant, only one of guns52 and 53 is provided.

[0070]FIG. 5 shows another embodiment of the invention, in which anode60 is hollow and has electron gun 61 centrally located with anode 60such as to produce an electron beam 62 which impinges on web 63 frombelow.

[0071]FIG. 6 shows a further embodiment of the invention in whichelectro-plating station 70 comprises a first electron beam gun 71positioned within anode 72 in similar fashion to FIG. 5, and a secondelectron beam gun 73 positioned above gun 71 and on the other side ofthe location of the target, being the respective part of web 74. Thus,the two beams 75 and 76 are directed and focused to collide in thevicinity of the target, and thereby reduce or eliminate the need orimportance of a seed layer.

[0072] The two beams may be finely focused and they may be of similarsize and directed in opposition.

[0073] The invention enables easy, precise and fast deposition ofmaterial onto a fast-moving web. Moreover, the invention enables theelimination of many of the components required in some electro-platingmachines, for example, seals and electrical-contacting fingers.

[0074] Electro-plating apparatus of the invention may be incorporated inan electro-plating machine comprising five in-line electro-platingstations, each being as described with reference to station S of theinvention. The web passes over each of the stations in turn so that anamoutn of copper is deposited on the web at each station. The currentdensity applied at each station can be set at an appropriate level forthe amount of copper to be deposited a that station as requried.

[0075] For example, in one electro-plating operation, it may beappropriate to supply a current density of about 50 Amps m⁻² at thefirst station to deposit a layer of copper thickness about 3.33×10⁻⁹meters, and then to apply a current density of about 300 Amps m⁻² ateach of the subsequent stations to deposit a layer of copper thicknessabout 2×10⁻⁸ meters in each. Such a current density profile may beappropriate, for example, to ensure that current in the tracks does notburn out in a typical electroplating operation, for example with the webrunning at about 0.2 metre min⁻¹.

[0076] In another electroplating operation, it may be appropriate toapply a current density profile which alternates between high and lowvalues and/or with time, for example to give varying depositionthickness or to change the copper characteristics.

[0077] A printed circuit 110 is shown in FIG. 7. The printed circuitcomprises a substrate 112 on which are printed a plurality of conductivetracks 114. The tracks 114 are printed onto the surface of the substrate112 using a screen printing technique, the screen printing ink beingprovided by a polymeric composition loaded with electrically conductivesilver particles and cured by exposure to ultra-violet light to providea pattern of discrete conductive tracks. The tracks as initially printedusing the UV curable ink are of relatively low conductivity.

[0078] In carrying out the illustrative method, of enhancing theconductivity of conductive tracks which have been produced by printingthem onto the substrate 112, the substrate carrying the printed tracks114 is coated with an electro-plating solution using a tool 116 which isindicated in chain dot line on FIG. 7 and which is showndiagrammatically from the end in FIG. 8, with part broken away.

[0079] The tool 116 comprises an absorptive member 118 provided by abody of flexible foam material having interconnecting pores. The tool116 further comprises a first electrode (not shown) which projects intothe absorptive member 118. The tool further comprises a pair of secondelectrodes 120 mounted at either side of the absorptive member 118,closely adjacent to a tip portion 122 of the absorptive member 118, butspaced from the tip portion 122 and electrically insulated from thefirst electrode and from the absorptive member.

[0080] Each of the second electrodes 120 is provided by a flexibleelectrically conductive blade member which is made of any suitable metalmaterial, for example, a copper alloy.

[0081] In carrying out the illustrative method which can be regarded asan specialized and innovative application of the know brush platingmethod, the absorptive member 118 is impregnated with an electro-platingsolution so that the first electrode makes electrical contact with thesolution. The tool 116 is then moved into contact with the surface ofthe substrate 112, with the tip region 122 of the absorptive member 118pressing lightly on the surface of the substrate 112 and with the secondelectrodes likewise lightly contacting the surface of the substrate 112.

[0082] After the tool 116 has been brought into contact with the surfaceof the substrate 112 it is wiped along the substrate 112 in thedirection indicated by the arrow A on FIGS. 7 and 8. As can be seenviewing FIG. 7, the tool 116 extends completely across the substrate116. Electro-plating solution is supplied to the absorptive member 118in sufficient quantity that, as the tool 116 is wiped across the surfaceof the substrate, a coating of electro-plating solution is wiped by thetool 116 across the surface of the substrate. The first electrode isconnected to a positive pole of an electroplating circuit to provide theanode, while both of the second electrodes 120 are connected to thenegative pole of the electro-plating-plating circuit to provide thecathode. Thus, as the tool 116 is wiped across the surface of thesubstrate, the conductive tracks 114 are electro-plated. The electricalcurrent supplied by the electro-plating circuit, in conjunction with thespeed of travel of the tool across the surface of the substrate,provides a control of the amount of electro-plating metal which isdeposited on the conductive tracks and is suitably controlled to providea layer of metal electro-plated on the conductive tracks, which is of adesired thickness, normally between about 10 and about 15 microns inthickness. Any appropriate electro-plating may be used, but a commoncopper sulphate electro-plating solution may be appropriate.

[0083] When the surface of the substrate 112 has been treated by thetool 116, any excess electro-plating solution may be rinsed from thesurface of the substrate 112, if necessary.

[0084] As the lines printed on to the substrate 112 by the curable inkmay be relatively fragile the pressure exerted by the tools 116 on thesubstrate should be very light, just sufficient to apply the necessaryelectro-plating solution and to make electrical contact between thesecond electrodes 120 and the conductive tracks 114.

[0085] The illustrative method provides a readily controlled method ofelectro-plating discrete conductive tracks carried on the surface of aprinted circuit substrate. As the substrate is not immersed in anelectro-plating bath, and the method is a substantially “dry” method ofelectro-plating, with only small amounts of electro-plating solutioncoming into contact with the substrate, it is possible to electro-platesubstrates which would be adversely affected by immersion in anelectro-plating bath. Furthermore, the method allows electro-plating ofdiscrete conductive tracks which has not hitherto been possible in anyconvenient manner.

1. A method of electroplating comprising: providing an anode current;applying an electron beam to a surface of a target in the vicinity ofthe anode current at the target; and passing electrolyte over thetarget, to thereby deposit material on the target.
 2. The methodaccording to claim 1, further comprising ionizing the target by treatingwith a scanning electron beam to create an opposite polarity to thepolarity of the anode.
 3. The method according to claim 1, furthercomprising providing an electrically conductive layer at the surface ofthe target.
 4. The method according to claim 1, further comprisingproviding a layer of material of greater density than that of aneighboring material.
 5. The method according to claim 1, furthercomprising providing material suitable for forming a lattice with thematerial being deposited.
 6. The method according to claim 1, furthercomprising focusing the electron beam by tuning of voltages appliedwithin a electron gun generating the electron beam.
 7. The methodaccording to claim 1, further comprising spreading the electron beam bytuning of voltages applied within an electron gun.
 8. The methodaccording to claim 1, further comprising varying a position of the beamon the target.
 9. The method according to claim 1, further comprisingdirecting a plurality of electron beams onto one or more regions of thetarget.
 10. The method according to claim 9, further comprisingdirecting two electron beams onto a target from opposite sides of thetarget.
 11. The method according to claim 10, further comprisingdirecting and focusing two electron beams, from opposite sides of atarget, to collide in the vicinity of a target.
 12. The method accordingto claim 9, further comprising directing two electron beams from oneside of the target, and directing one electron beam from the other sideof the target.
 13. The method according to claim 1, further comprisingdirecting an electron beam from a region of a source of the anodecurrent.
 14. Electro-plating apparatus comprising: an anode to providecurrent at a target, means to apply an electron beam to the targe; andmeans to pass electrolyte between said target and said anode, to therebydeposit material on the target.
 15. The apparatus according to claim 14,further comprising an electro-plating cell, wherein the cathodeconnection is formed by an electron beam.
 16. The apparatus according toclaim 14, further comprising a scanning electron beam gun to ionize atarget.
 17. The apparatus according to claim 14, further comprisingmeans to focus the electron beam by tuning of voltages applied within anelectron gun.
 18. The apparatus according to claim 14, furthercomprising means to spread the electron beam by tuning of voltagesapplied within an electron gun.
 19. The apparatus according to claim 14,further comprising means to vary a position of the electron beam at thetarget.
 20. The apparatus according to claim 14, further comprising anelectron beam source to direct a plurality of electron beams to one ormore regions of the target.
 21. The apparatus according to claim 14,further comprising an electron beam source to direct at least twoelectron beams which are on opposite sides of the target.
 22. Theapparatus according to claim 19, further comprising two electron beamguns positioned on opposite sides of the target for direction andfocusing of beams for collision at the target.
 23. The apparatusaccording to claim 14, further comprising one electron beam source todirect an electron beam from one side of the target, and two electronbeam sources to direct two electron beams from another side of thetarget.
 24. The apparatus according to claim 14, wherein an electronbeam source is located in the anode.
 25. A method of providing improvedconductive tracks on a printed circuit board, the printed circuit boardcomprising a substrate and conductive tracks printed on said substrate,by electro-plating said printed conductive tracks, comprising: coatingthe substrate carrying the printed conducted tracks with anelectro-plating solution with a tool which provides a first electrode ofan electro-plating circuit and a second electrode of saidelectro-plating circuit being provided by the printed conductive tracks,wherein the tool comprises an absorptive member in which the platingsolution can be carried, the first electrode being arranged to be inelectrical connection with the plating solution carried by theabsorptive member, and the coating of plating solution is applied to thesubstrate by wiping the absorptive member over the substrate, andwherein the tool also comprises a tool second electrode, electricallyinsulated from the tool also comprises a tool second electrode,electrically insulated from the first electrode and spaced from theabsorptive member, the tool second electrode being adapted to be wipedacross the surface of the substrate as the absorptive member is wipedacross the surface of the substrate and the tool second electrodecontacting the printed conductive tracks printed on the substrate toform the second electrode of the electro-plating circuit therewith,wherein the conductive tracks are treated with a scanning electron beamto ionize the tracks and create an opposite polarity to the polarity ofthe first electrode.